The present invention relates to a method and apparatus for transferring digital voice signals together with high speed data, and particularly to an all digital loop possessing a mode of low-power transfer of such signals. Additionally, the present invention relates to a telephone network comprising such apparatus.
With the proliferation of digital data transmission across telecommunications networks and inter-networks, it has become common practice to integrate the transmission of analogue telephone signals, associated with a xe2x80x98Plain Old Telephone Servicexe2x80x99 (POTS), with the transmission of other digital data, upon such networks (e.g. xDSL transmission). In many cases, the latter type of data may well be transmitted via the same transmission medium as the former signals (i.e. via a telephone line).
It is well known that analogue signals associated with POTS and, for example, the digital signal associated with an xDSL (Digital Subscriber Line) data stream, may be simultaneously transmitted via a twisted-pair transmission line.
Such simultaneous data transmission is clearly desirable and its realisation may typically involve the technique of frequency-division multiplexing of the two signals. According to such a technique, a low-frequency transmission band (i.e. 300 Hz-3400 Hz) is assigned to the transmission of analogue telephony signals and a high-frequency transmission band (i.e. band of high-frequency carriers modulated with signal data) is assigned to transmission of the xDSL data stream. Consequently, simultaneous transmission of digital xDSL data and independent analogue telephone signals may be provided via the same twisted-pair transmission line of a telecommunications network.
Such a transmission technique is known, generally, as xe2x80x98out of bandxe2x80x99 transmission since telephone signals are transmitted outside of the xDSL transmission band.
A recognised deficiency of this technique lies in the necessity of employing suitable multiplexing/de-multiplexing equipment capable of accepting the relatively high voltage levels typically present in analogue telephone signals (e.g. the ringing signal). Consequently, this equipment (POTS splitter) is often prohibitively expensive (operator dependent) and/or bulky. Born of this realisation came the incentive to develop systems for the simultaneous transmission of digitized telephone signals, together with other independent xDSL digital data, over a twisted pair telephone line.
Such a system, known as an xe2x80x98all digital loopxe2x80x99, thereby obviates the need to employ such expensive/bulky equipment by transmitting telephone signals, together with other digital data, in a digital format only and thereby removing the high-voltage components associated with the analogue telephone signal. For example, digitized telephone signals may be embedded within the frames associated with an xDSL data stream. In such a case, xe2x80x98in-bondxe2x80x99 transmission is achieved since the digitized telephone signals are thereby transmitted within the same transmission band (of carriers) as the xDSL data stream.
In the case of ADSL or VDSL (Very High Speed Digital Subscriber Line) this implies that, in contrast to the traditional xe2x80x98overlay scenarioxe2x80x99 where POTS or ISDN are transported xe2x80x98out of bandxe2x80x99, the telephony signals are transmitted xe2x80x98in-bandxe2x80x99. That is to say, analogue telephone signals originating from a telephone set are transmitted therefrom to an xDSL network termination node containing xDSL equipment operable to digitise and subsequently transmit those signals to an xDSL line termination in the access node providing the gateway to the PSTN (Publicly Switched Telephone Network) network. This is one way to realise the all digital loop. Alternatively, the telephone signals may be digitized before being supplied to an xDSL network termination to be transmitted subsequently over the all digital loop.
However, a potential weakness of an approach such as this lies in the need to digitise the telephone signals and embed them, via the xDSL equipment, into the xDSL data stream prior to simultaneous transmission thereof. Should xDSL equipment fail due, for example, to a loss of power then both xDSL and (more importantly) telephone communications along the all digital loop would break down.
Clearly, such a loss in telephone communications would be inconvenient at the very least and, conceivably, precipitate disastrous consequences should POTS telephone services be urgently required. Thus, the provision of an emergency xe2x80x98lifelinexe2x80x99 within such all digital loops, which would guarantee continued telephone communications services (POTS) is therefore paramount.
A common prior art technique of providing continued POTS under such circumstances involves the step of reverting to independent analogue transmission of telephone signals. Analogue telephone signals (or digital telephone signals via for instance an ISDN interface) are received at a telephone network termination (NT) node (from a telephone set) for subsequent digital transmission therefrom to a line termination (LT) access node providing a gateway to the PSTN network. Under normal operating conditions xe2x80x98in-bandxe2x80x99 transmission of those telephone signals with digital data streams simultaneously input to the xDSL equipment (at the NT node), would then take place in accordance with all digital loop transmission techniques as outlined above.
As discussed above, in the event of power failure at the NT node of the digital loop, xe2x80x98in-bandxe2x80x99 digital transmission of neither the telephone signal nor the xDSL data could take place and POTS would be lost. However, when failure occurs, a series of switches may be switched at each node of the digital loop (i.e. the NT and LT nodes) so as to provide a bypass circuit which permits the analogue/digital telephone signal (input to the NT node) to bypass all xDSL equipment in the respective node. Consequently, a direct POTS lifeline from the NT node to the LT node, and thence to the PSTN, is thereby provided.
Unfortunately, as will be readily apparent, such a system requires the use of additional circuitry to bypass the xDSL equipment at both nodes. Furthermore, old existing linecards are required at the LT (access) node to operate the additional circuitry. These linecards must be kept in place at all times so as to guarantee a POTS lifeline and, therefore, cannot be replaced with other equipment should the need, or desire, arise.
The present invention aims to overcome at least some of the above identified deficiencies of the prior art by providing an all digital loop in which a POTS lifeline is provided without recourse to switching/bypass circuitry and the corresponding linecards associated therewith.
Accordingly, in a first of its aspects the present invention provides apparatus for use in a digital telecommunications network, the apparatus including:
a first network node;
a second network node; and
a data transmission line connecting the first network node to the second network node;
wherein each one of said first and second network nodes is operable to transmit to the other of said first and second network nodes, at least one digitized telephone signal with other digital data via a set of carriers, the apparatus being characterised in that:
one of said first and second said network nodes includes or is associated with means which in the event of power failure at the first said network node, is operable selectively to provide power to the first node such that the first node operates only via a limited subset of said carriers which limited subset is such as to minimize the power required for the transmission thereof so as to provide transmission of a telephone lifeline signal between the first and second nodes.
Thus, it will be understood that the present invention, in its first aspect, aims to guarantee a POTS lifeline by means of providing an emergency power supply operable to selectively power the continued digital transmission of telephone signals between two nodes of e.g. an all digital loop using minimal power. Preferably, the limited subset of carriers provides transmission of only one telephone voice channel in the telephone lifeline.
Ideally, the means operable to selectively provide power to the first node includes a power source located substantially at the second node and power is selectively provided to the first node remotely from the second node. However, a power source located substantially locally with first node could be used to selectively provide to the first node locally. For example, the local power supply could include a UPS (Uninteruptable Power Supply) system, a battery of a super-capacitor or the like.
Preferably, said at least one digitized telephone signal is in-band transmitted with said other digital data using e.g. digital subscriber line (xDSL) transmission techniques. Preferably, the other digital data with which the digital telephone signal data is transmitted comprises a digital subscriber line data stream (xDSL), and former is transmitted in-band with the latter by embedding the former into the data frames of the latter. ADSL may be used.
Consequently, it will be seen that the first node may be located at an xDSL subscriber premises and the second node may be located at an xDSL central office.
According to a second aspect of the present invention, there is provided a central office of a telecommunications network, having an all digital loop including:
a first network node;
a second network node located at the central office; and
a data transmission line connecting the first network node to the second network node;
wherein each one of said first and second network nodes is operable to transmit to the other of said first and second network nodes, at least one digitized telephone signal with other digital data via a set of carriers, the all digital loop being characterised in that:
one of said first and second said network nodes includes or is associated with means which in the event of power failure at the first said network node, is operable selectively to provide power to the first node such that the first node operates only via a limited subset of said carriers which limited subset is such as to minimize the power required for the transmission thereof so as to provide transmission of a telephone lifeline signal between the first and second nodes.
According to a third aspect of the present invention, there is provided a customer premises for use with an all digital loop of a telecommunications network, the all digital loop including:
a first network node located at the customer premises;
a second network node; and
a data transmission line connecting the first network node to the second network node;
wherein each one of said first and second network nodes is operable to transmit to the other of said first and second network nodes, at least one digitized telephone signal with other digital data via a set of carriers, the all digital loop being characterised in that:
one of said first and second said network nodes includes or is associated with means which in the event of power failure at the first said network node, is operable selectively to provide power to the first node such that the first node operates only via a limited subset of said carriers which limited subset is such as to minimize the power required for the transmission thereof so as to provide transmission of a telephone lifeline signal between the first and second nodes.
According to a fourth of its aspects, the present invention provides a method of transmitting digital data via a digital telecommunications network, the method including the steps of:
providing a first network node;
providing a second network node;
providing a data transmission line and connecting the first network node to the second network node therewith; and
transmitting from any one of said first and second network node to the other of said first and second network nodes, at least one digitized telephone signal with other digital data via a set of carriers; the method being characterised in that in the event of power failure at the first said network node, the method includes the further steps of:
selectively providing power to the first node such that the first node operates only via a limited subset of said carriers which limited subset is such as to minimise the power required for the transmission thereof so as to provide transmission of a telephone lifeline signal between the first and second nodes.
Thus, it will be understood that the present invention, in its fourth aspect, aims to guarantee a POTS lifeline by means of providing an emergency power supply operable to selectively power the continued digital transmission of telephone signals between two nodes of e.g. an all digital loop using minimal power. Preferably, the limited subset of carriers provides transmission of only one telephone voice channel in the telephone lifeline.
Power may be provided locally to the first node (e.g. via a power supply located at the customer (subscriber) premises) using e.g. a UPS, a battery of a super-capacitor or the like, or may be remotely fed from the second node.
Preferably, the at least one digitized telephone signal is in-band transmitted with said other digital data using e.g. digital subscriber line (xDSL) transmission techniques, such as ADSL.
Consequently, the first node may be located at an xDSL subscriber premises and the second node may be located at an xDSL central office.